1. Field of the Invention
The present invention relates to improving the accuracy and reliability of transmission of free-space optical digital communication signals. More particularly, the present invention is directed to a system and method for synchronizing transmitted optical digital communication signals by simultaneously transmitting a radio frequency timing signal.
2. Description of the Related Art
Wireless digital communication has been typically implemented through the use of radio frequency (hereinafter "RF") communication systems. RF systems have the advantages of range, reliability and relatively low cost. However, the available RF spectrum is quite limited due to FCC regulations. Furthermore, RF communication generates and is affected by electromagnetic noise. As a result, in recent years various alternatives to RF communication, for least short-range applications, have been developed. One such technology, optical free-space digital communication, has begun to gain recognition due to the advantages it offers over RF communication. Unlike radio frequency communication systems, optical free-space digital communication systems do not cause and are not affected by electromagnetic interference and are not regulated by the FCC. Optical communication systems use infrared ("IR") spectrum light to transmit and receive digital information between a transmitter and a receiver on an optical carrier in the form of digital pulses stored in a sequence of "frames". Most commonly, the relative position of each digital pulse within its frame is encoded in a binary "word" as a sequence of binary digits. Each word is transmitted by the transmitter along with its corresponding frame. The words serve as synchronization signals for transmitter-receiver synchronization by enabling the receiver to determine the position of each digital pulse within its frame and to anticipate the next digital pulse, thus improving the accuracy of optical free-space communication. In a typical optical digital communication system, both the digital pulses and the timing base of the frames (i.e., the words) are carried by optical radiation. An example of such an arrangement is a PPM or Pulse Position Modulation system, in which a single digital pulse is transmitted within each frame, and the position of each pulse relative to the boundaries of its frame is encoded in a binary word that is transmitted along with each frame. Each word is then decoded by the receiver so as to locate and acquire the digital pulse within the word's corresponding frame.
Optical free-space digital communication systems, however, suffer from several significant disadvantages. For example, ambient optical noise, such as proximal light sources, can significantly affect a system's accuracy and reliability by distorting the optical signal and making transmitter-receiver synchronization difficult to achieve. In the presence of ambient optical noise, the receiver often encounters difficulties in accurately determining the boundaries of each frame in the optical carrier signal, whereby some digital pulses may be misinterpreted or lost. Furthermore, mobile and stationary objects that obstruct an optical signal can further disrupt the synchronization process. Finally, optical signals quickly attenuate in free space, thus further distorting the signal received by the receiver.
Various error-correction techniques have been developed to reconstruct a received optical carrier at the receiver. While digital pulses can be reconstructed with some effort, it is very difficult to reconstruct timing or frame definition signals of a carrier, and thus the positions of the digital pulses, with sufficient accuracy.
It would thus be desirable to provide a system and method for optically transmitting digital data along with a synchronization signal which is unaffected by ambient optical noise and by obstructing objects.